Navien furnace displays E302, unit may not start after power outage

Safety: Turn off power at the breaker before handling control boards.

• Power cycle: Turn off the breaker for at least 5–10 minutes to allow capacitors to discharge, then restore power and see if the error clears.
• Check supply voltage: Use a multimeter to verify that line voltage to the furnace is within Navien's specified range and stable. Have an electrician correct any supply issues.
• Factory reset (if supported): Some Navien models allow a service-level reset of configuration parameters. Refer to the service manual and only perform this if you understand the implications.
• Board replacement: If E302 persists, the main PCB's memory is likely corrupted. Replacement with a properly programmed Navien OEM board by an authorized technician is required.